There are a number of standard voltage controlled oscillators, analog and digital, that have been used in a variety of applications.
FIG. 1 shows a typical continuous time analog VCO. The circuit is made up of two variable current sources I1 and I2, a capacitor C which functions as a storage element, and a comparator with hysteresis such as a Schmitt trigger 10. The capacitor C is charged from the current source Il until it exceeds the high trip voltage, Vhigh of the Schmitt trigger 10. When Vhigh is reached, the capacitor is switched to I2 and discharged until the low trip voltage, Vlow of the Schmitt trigger is reached at which time the capacitor is again charged from the current source, Il, and the process repeats. Assuming I2=I1, the frequency of the oscillation F equals: EQU F =0.5*I1/[C1*(Vhigh-Vlow)]
From the equation, the frequency of the oscillator is directly dependent on the variable current source, Il. Because the current source is varied by a voltage, Vcontrol, the oscillator frequency is also controlled by this voltage, hence the designation voltage controlled oscillator. The absolute value of the current source is usually dependent on a resistor; therefore, the accuracy of the frequency of the VCO is dependent on a RC product. The absolute accuracy of integrated resistors and capacitors is very poor so external components are required. Accurate external components are expensive and temperature dependent which limit absolute accuracy over temperature and time to approximately 2 percent. This basic circuit approach is therefore unacceptable for applications where greater accuracy is required.
Another type of VCO is one comprised of switched capacitor circuits. It is similar to the linear VCO previously described except the current source and capacitor are replaced by a switched capacitor integrator with discrete time updates. This circuit has continuous voltage output, but it has a discrete time output. A typical circuit is shown in FIG. 2. Vcontrol is applied to a switched capacitor integrator generally designated 12 having a time constant which is determined by the ratio of CA to CB and the input clock rate. The value of Vcontrol determines the time it takes for the integrator to ramp to a known voltage, Vhigh or Vlow in the Schmitt trigger 14. Input clock phasing, determined by the state of the switches S5-S8, controls whether the integrator has a positive or negative gain. In one state .phi.1 is applied to S1 and .phi.2 is applied to S2. In the other state .phi.2 is applied to S1 and .phi.1 is applied to S2. When the integrator toggles the Schmitt trigger 14, the input phasing is switched. This causes the integrator to ramp toward the other trip point where the process repeats itself. Thus, an oscillator is created whose frequency is directly dependent on the input control voltage.
The accuracy of this circuit is dependent on how accurately the capacitors CA and CB are matched and the accuracy of the clock. The clock is established by an external crystal and is not the main source of error. The capacitor matching ratio is limited to approximately 0.1 percent, if the capacitor ratios are within a factor of 10. The SC VCO's output frequency is theoretically perfect because the integrator has no voltage quantization; however, there is a maximum phase error due to the discrete time nature of the circuit. The phase error can be minimized by having a large number of steps in the ramp voltage. This can only be achieved with a ratio of CA to CB which is very large. The greater the mismatch in size of CA and CB, the less the overall accuracy of the oscillator. In a practical circuit, the desired maximum phase error would require an unreasonably large capacitor ratio which would limit the accuracy to approximately 0.3 percent or greater and be very area inefficient.
Another type of VCO is fully digital. In patent No. 4,577,163, for example, a digital PLL with a DCO is presented. In this circuit, the output frequency is controlled by an input bit pattern. The input bit pattern directly affects the value of the frequency. The circuitry is both discrete in time and in magnitude (a finite word length takes the place of the input control voltage). This has the problem of phase error and center frequency accuracy The phase error is a fundamental limitation in a discrete time system. This can be minimized in a digital system by making the clock to center frequency ratio high. It is fully digital, so the capacitor ratio problem of the SC VCO is eliminated, and digital circuitry requires a relatively small area. In this circuit the output frequency is controlled by a finite input word length. This will cause the center frequency to vary over time. This inaccuracy is unacceptable in some applications.